This invention relates to photoflash lamps and, more particularly, to test flashlamps useful for the test and development of lamp containment systems.
A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible metal located in the envelope, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. In lamps intended for battery operated flash systems, the envelope also includes an electrical ignition system comprising a tungsten filament supported on a pair of lead-in wires having a quantity of ignition paste on the inner ends thereof adjacent to the filament. This type of lamp is operated by the passage of an electrical current through the lead-in wires which incandesces the filament to ignite the ignition paste which in turn ignites the combustible metal in the envelope. In the case of percussive-type photoflash lamps, such as described in U.S. Pat. No. 3,535,063, a mechanical primer is sealed in one end of the lamp envelope. The primer may comprise a metal tube extending from the lamp envelope and a charge of fulminating material on an anvil wire supported in the tube. Operation of the percussive photoflash lamp is initiated by an impact onto the tube to cause deflagration of the fulminating material up through the tube to ignite the combustible metal disposed in the lamp envelope.
During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal or metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment is a problem. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the envelope, such as cellulose acetate.
For customer convenience, and in keeping with the miniaturization of popular cameras, flashlamps have evolved to ever-smaller sizes. For example, subminiature flashlamps having an envelope volume of less than one cubic centimeter are presently mass produced in large quantities for use in small photographic flashlamp units referred to as flashcubes. As described in U.S. Pat. No. 3,244,087, electrical flashlamp units of this type comprise: a container having a plurality of closed transparent sides; a plurality of reflectors disposed in the container, one along each side thereof; and a photoflash lamp disposed in operative relationship with respect to each of the reflectors. Percussive-type lamps are employed in multilamp cubical units having respective preenergized striker springs for each lamp as described in U.S. Pat. No. 3,597,604.
The light output of a flashlamp is directly related to the quantity of oxygen contained in the lamp; hence, the reduction of lamp size has been paralleled by increasing oxygen fill pressures. In addition, higher combustible metal fill weights are employed per unit of envelope volume. These higher internal pressures and fill weights per unit volume require stronger lamp envelopes and coatings, and stronger transparent containers for multilamp units.
One significant problem in the development of improved lamp containment systems is that the incidence of lamps that fail to contain is extremely low. Even though changes such as excess oxygen damaged glass or vapor weakening of the lamp coatings are introduced, the number of lamps required for testing, in order to get meaningful measures of relative containment with different proposed new coatings and containers, is prohibitively large.
In consideration of lamp reinforcing coatings, by way of example, it is known that coating tensile strength, impact strength, and heat distortion temperature are all significant parameters. It has been found, however, that knowledge of standard test method values for these characteristics does not permit reliable estimation of the relative containment capability of a proposed lamp coating. The problem is that tensile, impact, and thermal stressing of a lamp coating occur simultaneously and that the mechanical properties vary as a continuous function of both temperature and rate of load application. Meaningful data must come from evaluations carried out under actual operating or usage conditions. Studies of this type have previously required tests based on tens or hundreds of thousands of lamps. Even with such large test groups, the results were often not conclusive.